This invention relates to a method of forming an integral sheet metal rivet.
Integral rivets have long been known to attach objects of various kinds to metal panels. In recent times, one of the most widely used applications for an integral rivet has been to attach opening levers or tabs to the scored ends of beverage cans. One of the earliest patents directed to a method for forming a rivet for such an application is Fraze U.S. Pat. No. 3,191,564. In the first step of Fraze's method, a bubble having an area in plan substantially greater than area in plan of the desired rivet is formed by stretching the sheet without making any substantial reduction in thickness. The bubble is then reshaped into a rivet having a hat-shaped configuration with the transverse wall of the hat section made substantially thicker during reshaping and the rivet having a much smaller area in plan. A tab or lever is then attached to the can by staking the rivet. In staking the rivet, it is positioned on an anvil support and the transverse wall is impacted with an opposing anvil or hammer to extrude metal radially outwardly to form a flange which holds the tab against the can end.
Brown U.S. Pat. No. 3,479,733 describes a rivet forming method whereby the can end is supported on a flat die and impacted with an opposing die having an opening therein. The opposing die has an annular portion projecting outwardly from its face around the opening so that when it impacts the end metal, metal is squeezed inwardly to form a bubble. To attach a tab, the tab is positioned over the bubble and the bubble is simply mashed or compressed downwardly to form a flange which impinges against the tab and holds it in place on the can end.
Brown U.S. Pat. No. 3,638,597 describes yet another method for forming a rivet. An initial dimple or bubble is formed by a combination of stretching and squeezing the metal. The sheet to be formed is placed over a die cavity and contacted with a punch which stretches the sheet into the cavity. As the punch nears the end of its stroke, metal between the punch and walls of the die cavity is squeezed and extruded radially into the cavity to thicken the crown portion of the dimple which is later staked to form the flange.
The foregoing are examples of many patents which discuss forming integral rivets to attach devices to can ends. Each of the described rivets is suggested for use in rigidly attaching relatively thin metal objects to can ends. The shanks of these rivets are all relatively short, therefore, to accommodate the thin metal. Furthermore, there is no need for the rivet head to have more than a relatively narrow outwardly extending flange since the object is rigidly attached and is held in place by the rivet shank tightly filling the opening in the object as well as the outwardly extending flange covering the object adjacent the opening. The high modulus of elasticity of the metal object provides a high resistance to enlargement of the rivet hole in it and a narrow flange, therefore, provides a secure attachment. Although the known rivets and methods for making them have been suitable for attaching thin metal objects, such as tabs to can ends, the attachment of at least some other kinds of objects presents a totally different set of problems and conditions. For exam application Ser. No. 738,975 filed May 29, 1985, now U.S. Pat. No. 4,580,692 rotatably attaching a plastic cap to a can end. It is approximately twice as thick as the typical metal tab presently being attached to can ends with integral rivets; thus, the rivet requires a shank twice as long. Furthermore, the plastic cap is rotatable about the rivet shank. The rivet must be capable of being staked to form the head, therefore, without expanding the shank or impinging the head against the cap so tightly that it cannot be rotated. Since attachment of the cap is solely dependent upon the flange overlying the cap around the opening and plastic is readily deformable because its modulus of elasticity is approximately 1/500 that of a typical aluminum tab, it would be desirable to extend the rivet flange outwardly from the shank significantly further than rivets known heretofore. Because the rivet hole in the cap has a very low resistance against enlargement, it is also desirable to bend the flange downward to interlock the cap with the rivet. By bending the flange downward, a cap retaining cavity underlying the flange can be formed which has at least a portion having a dimension parallel to the rivet shank greater than the dimension at the entrance to the cavity. Thus, if the cap is designed to have a portion which substantially fills such a cavity, the cap cannot be disassembled without subjecting the plastic to a large percentage compressive deformation. For all of the foregoing reasons, it would be desirable to have an improved rivet and method of making it, particularly for attaching a deformable plastic object to a can end.